The evaluation of posterior cruciate ligament injury of the knee should be objective. Then stress machine, Gonylaxometer, knee ligament arthrometer (K-T 1000) and Genucome were introduced recently, but they have many problems such as soft tissue error, expensiveness and low popularity. Authors modified the radiological assessment of anterior cruciate ligament injury, reported by Hooper(1986), and applied to the evaluation of posterior cruciate ligament injury. The patient was positioned supine with the one side of leg held on a support so that the knee was flexed approximately 45°to 60°. The thigh was unsupported and a weight(0.9 Kg to 4.5 Kg with increment of 0.9 Kg) was placed on the anterior leg (just distal to the patellar tendon) and lateral view had been taken in this stressed position. Same procedure was done on the other side and the posterior displacement of both knees were compaired. The differences of posterior displacement between both knees of 50 normal adults were studied and 9 cases of posterior cruciate ligament injury were studied, too. The following results were obtained. 1. The difference of posterior displacement could be measured objectively and it is useful for the diagnosis of posterior cruciate ligament injury and the objective evaluation after conservative or operative treatment of posterior cruciate ligament injury patient. 2. The differences of posterior displacement between both knees of 50 normal adults were ranged from 0 to 3 mm with a mean of from 0.88 ±0.87mm to 1.14 ±0.94mm. 3. The differences of posterior displacement between injured knee and normal knee of posterior cruciate ligament injury patient were more than 6mm in 9 patients. 4. The radiological method was inexpensive and easy to perform.